Local Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow

Abstract

We perform 3D vertically-stratified local shearing-box ideal MHD simulations of the magnetorotational instability (MRI) that include a net vertical magnetic flux, which is characterized by beta_0 (ratio of gas pressure to magnetic pressure of the net vertical field at midplane). We have considered beta_0=10^2, 10^3 and 10^4 and in the first two cases the most unstable linear MRI modes are well resolved in the simulations. We find that the behavior of the MRI turbulence strongly depends on beta_0: The radial transport of angular momentum increases with net vertical flux, achieving alpha=0.08 for beta_0=10^4 and alpha>1.0 for beta_0=100, where alpha is the Shakura-Sunyaev parameter. A critical value lies at beta_0=10^3: For beta_0>10^3, the disk consists of a gas pressure dominated midplane and a magnetically dominated corona. The turbulent strength increases with net flux, and angular momentum transport is dominated by turbulent fluctuations. The magnetic dynamo that leads to cyclic flips of large-scale fields still exists, but becomes more sporadic as net flux increases. For beta_0<10^3, the entire disk becomes magnetic dominated. The turbulent strength saturates, and the magnetic dynamo is quenched. Stronger large-scale fields are generated with increasing net flux, which dominates angular momentum transport. A strong outflow is launched from the disk by the magnetocentrifugal mechanism, and the mass flux increases linearly with net vertical flux and shows sign of saturation at beta_0=10^2. However, the outflow is unlikely to be directly connected to a global wind: for beta_0>10^3, the large-scale field has no permanent bending direction due to dynamo activities, while for beta_0<10^3, the outflows from the top and bottom sides of the disk bend towards opposite directions, inconsistent with a physical disk wind geometry. Global simulations are needed to address the fate of the outflow.